The invention described herein relates to a method for selecting hollow microspheres useful as containers for the high pressure gaseous deuterium and tritium fuel in laser fusion targets.
A mixture of deuterium and tritium is a preferred fuel for laser fusion, primarily because the least energy is required to cause these two isotopes to undergo thermonuclear reaction. It is desirable that the DT mixture acted upon by the laser radiation be as dense as possible. The optimum density is achieved by cooling the mixture sufficiently that it becomes a solid. This, however, requires temperatures below 20 K which imposes very severe constraints not only on the manufacture, but also on the handling of laser fusion targets.
Alternatively, hollow, spherical, DT-gas-filled targets with diameters ranging from 30 to greater than 200 .mu.m and with contained fuel pressures varying from 10 to 1000 atm (at 298 K) are of interest for laser fusion. The primary gas-containment vessels of these targets are hollow microspheres. The targets are filled by diffusing DT fuel gas through the walls at elevated temperatures, taking advantage of the exponential temperature dependence of the permeability to allow the gas to be retained for useful times at room temperature. Thus, when the hollow microspheres are placed in a deuterium and tritium gas mixture of a desired ratio at high pressure and elevated temperature, the deuterium and tritium readily enter the hollow microspheres and equilibrate to the surrounding gas pressure. When the hollow microspheres are cooled to room temperature, the diffusion rate through their walls is greatly reduced, so that the DT mixture within the hollow microspheres remains at high pressure for times which permit useful storage before the targets are irradiated by the laser.
An essential requirement of the hollow microspheres is that they have a thin and uniform wall thickness. Hollow microspheres that are aspherical or have nonuniform walls or walls with defects therein are not suitable. Hollow microspheres of interest include metal, ceramic, plastic, and glass.
Various types of hollow microspheres are commercially available. Thus, for example, nickel-alloy hollow microspheres sold under the tradename Solacells by the Solar Division of International Harvester, have a composition by weight of .about. 70% Ni, 21% Mn, 2.5% Si, 1.5% each of Fe and B, and trace quantities of numerous other metals. Solacells are available in sizes from 75 .mu.m to greater than 500 .mu.m diameter, with wall thicknesses of 0.8 to 2.5 .mu.m.
Glass hollow microspheres may be obtained under the tradename Eccospheres or Microballoons from Emerson and Cuming Company. Eccospheres are available in many different grades. The IG-101 grade is a soft soda glass consisting by weight of .about. 78% SiO.sub.2, 3% B.sub.2 O.sub.3, and 19% Na.sub.2 O. The SI grade is a borosilicate glass consisting by weight of .about. 92% SiO.sub.2, 2.5% B.sub.2 O.sub.3, 2.5% Na.sub.2 O, and with the balance unknown. The size of the Eccospheres range from &lt; 40 .mu.m through 200 .mu.m, with wall thicknesses of &lt; 1 to &gt; 2 .mu.m.
Glass microspheres designated 3M microspheres are available in many types from Minnesota Mining and Manufacturing Company. The B40A type is made from a soda-lime glass consisting by weight of .about. 78% SiO.sub.2, 11% Na.sub.2 O, 7% CaO, and 4% B.sub.2 O.sub.3. It has about the same size range as the Eccospheres and has wall thicknesses ranging from about &lt; 1 to &gt; 3 .mu.m.
Unfortunately, no technique has as yet been devised for manufacturing these hollow microspheres such that they routinely will meet the strict size and wall thickness requirements for use in laser fusion targets. It is thus necessary to perfect techniques for separating from a much larger mass of imperfect hollow microspheres those hollow microspheres which do in fact conform to the requisite size and wall thickness. Thus, for example, the glass hollow microspheres are typically available in minimum orders of 2 to 10 lb. There are about 10.sup.10 hollow microspheres per pound, and it is conservatively estimated that only .about. 5 in 10.sup.5 is suitable for use in a laser fusion target. The problem is selecting the sixty thousand good hollow microspheres from the ten billion bad ones in each pound.